This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. ABSTRACT: An important application of cryo-electron tomography is to identify and determine the orientation and conformation of large macromolecular assemblies in their native cellular environment. This requires the development of algorithms that can efficiently detect target structural motifs within noisy tomographic 3-D density maps, using probe structures provided by single-particle cryo-EM or x-ray crystallography. Dr. Renken is continuing to work on improved methods for motif search using RAMOS (Rapid Motif Search: Rath et al., 2004, J. Struct. Biol. 145, 84-90). RAMOS is an algorithm that uses local variance in order to calculate the locally normalized cross-correlation function between a small motif and a larger volume. The method is being tested using tomographic reconstructions of frozen-hydrated triad junctions from skeletal muscle. The challenge is to determine the location and orientation of ryanodine receptors (RyR) on vesicles of sarcoplasmic reticulum (SR). The motif used for searching is a 3-nm-resolution map of the RyR derived from single-particle reconstruction.